When a flange portion is formed on a metal sheet by press forming along a projecting outer edge a part of which projects outward, there are cases in which shrinkage deformation arises in an end portion of the projecting portion of the flange portion. This is referred to as shrink-flanging. In the case of minor shrink-flanging, its influence is limited to only a phenomenon of an increase in sheet thickness. However, when the amount of shrink-flanging increases, wrinkles are formed in the course of press forming and the wrinkles remain after press forming.
The wrinkles are undesirable because they can cause shape defects of press formed products and wastage of dies. In particular, when the shrinkage is intense and large wrinkles are formed, the wrinkles further cause cracks of press forming on the metal sheet. Various methods have been developed that avoid the forming of wrinkles caused by such a shrink flange. For example, Patent Literature 1 discloses a method that facilitates compressive forming of a shrink flange portion by providing through-holes in the shrink flange portion in advance. Patent Literature 2 discloses a method of preventing wrinkles by clamping, at least at an early phase of press forming, a part of a press formed product, on which shrink-flanging is performed, at near its end portion with a U-shaped block and performing press forming afterward. Patent Literature 3 discloses a method of dispersing deformation by elaborating the shape of the end portion of a bending tool.